This invention relates to apparatus for taking samples, especially of water, from boreholes in the ground.
For taking samples from several different levels or depths within the name borehole, it is conventional to pass a length of continuous multi-channel (CMC) tubing down into the borehole. CMC tubing comprises a plastic (polyethylene) extrusion, in which a number of conduits, running the length of the CMC tubing, are isolated from each other by extruded internal walls. Typically, CMC tubing comprises six outer conduits, each having a pie-shaped cross-sectional profile, which surround a central conduit of hexagonal profile, making a total of seven conduits. Thus, samples can be taken from up to seven different depths.
In order to take a sample from the borehole from a particular depth, other than the lowermost depth, it is necessary to make a water-entry port in the side of the CMC tubing at that depth, and for the port to connect into one of the conduits, whereby water can pass, through the port, into that conduit.
One of the aspects that must be addressed by the designer, when using CMC tubing, lies in the seal that must be provided, to seal off the portion of the conduit that lies above and/or below the port. In most cases, in order to transport the sample to the surface, the conduit must be pressurised (from the surface), and the seal in the conduit must be able to withstand the pressure involved, without becoming dislodged and sliding lengthwise along the conduit.
Anchoring the seal into the conduit, so the seal does not slide along the conduit, is all the more difficult because the CMC tubing is extruded in polyethylene, which has a low coefficient of friction. Also, the CMC tubing is not very rigid, whereby the tubing can become distorted if a seal is mechanically forced into one of the conduits too tightly. Furthermore, extruding polyethylene does not produce very accurately repeatable profiles.
Thus, the designer of the seals for the conduits in CMC tubing must have it in mind that the conduits are characterised by: low friction; walls that lack solidity; a non-round, angular shape; and only moderate precision.
One of the ways in which the conduit seals have traditionally been made is by filling the segment-shaped conduit with glue, or by glueing a plug into the conduit. However, the conduit seals are (usually) made in-situ, i.e in the field, at the site of the well into which the CMC tubing is to be installed. The usual procedure is that the CMC tubing is laid out (horizontally) on the ground, for the various ports, packers, seals, etc to be made and assembled, at the appropriate locations along the horizontal length of the tubing (i.e along what will be the vertical depth of the tubing when it is installed in the borehole).
While it is easy to manufacture properly effective seals, with quality-control inspection to ensure high standards, in a factory, it is all too possible for field-installed glue-based seals to be inadequate. It is hardly practical to field-test the seals for pressure tightness, and what might look to the installation technician to be a sound seal might be blown out by the first application of pressure to the conduit. And the technician might not even know that the seal had been blown out, if the seal moved in a way that left it still possible to extract a sample of water from the borehole. Consequently, even if all the glue-based seals were perfect, it would not be possible to place much reliance on that being so. Sometimes, for example when assessing the location of a contaminant spill in groundwater, the results of the sampling are examined by a tribunal, which must be assured that the samples have indeed come from the designated depth.
The invention is aimed at providing a sealing system for the conduits of CMC tubing, in which such difficulties are not significant.
The invention addresses the difficulty of forming a seal in e.g a segment- or pie-shaped conduit, or in a hexagon-shaped, or other non-round, or angular, profiled conduit, in a manner that is highly cost-effective in the context of extracting water samples from a borehole. The scope of the invention is defined by the accompanying claims.